It has hitherto been attempted to solve these problems in the following manner.
The most widespread cleaning technique consists in using a jet of pressurised water, with possibly the addition of appropriate cleaning products.
The upper face of the perforated platform is cleaned automatically, for example by booms or rotary sprays which are connected to the system with raking arms and blades and also ensure cleaning of the central shaft possibly provided in order to support and guide the raking system vertically, as well as of the walls of the tank. The most difficult part to clean is the space situated between the bottom and the perforated platform, in particular the lower face of the latter, as well as the elements of the carrying structure.
In order to solve this problem, the most widely used solution consists in positioning the perforated platform by construction, that is to say permanently, at the height of a man, and in having the cleaning performed by an operator using high-pressure equipment. This work is time-consuming, difficult and dangerous (slippery floor) and its quality depends on the operator. It must be added to this that the carrying structure of the platform rests on props in order to support the weight of the grain. The operator must therefore move among a multitude of props, which moreover hinders automatization using robotic means.
This solution has the additional drawback of requiring a very large volume of water in order to fill the space between the bottom and the perforated platform; the consumption of water is thus increased without any gain as regards the treatment of the grain.
Another solution has therefore been envisaged, consisting in providing a vertically movable perforated platform, and in raising it only at the time of cleaning, this platform thus having, during normal operation, its usual height above the bottom of the tank. Thus, the height beneath the perforated platform is reduced to its minimum, which, during steeping, allows the usual characteristics of the process to be retained, in particular the CO.sub.2 extraction rate (8 to 10 m.sup.3 /s) and the flowrate of compressed air (180 to 200 mm head of water). This solution requires however the introduction beneath the platform of actuation means for raising it; these actuation means are then submerged in the steeping water and may constitute zones of accumulation of contaminating material which are hard to clean. It is moreover necessary to introduce a fluid beneath the perforated platform (electricity, compressed air, hydraulic fluid, etc.) in order to supply the aforementioned actuation means, which constitutes an additional drawback.